[Lucika]

AFAIK the German FS2 is currently not compatible with FSO, am I right? I never had a German version, but I am currently learning the language so I've got interested. Are there any German campaigns perhaps?

[FUBAR-BDHR]

I believe it is compatible with FSO but not with TCs and perhaps some mods.  The problem lies in the language being stored in the registry and that is accessed by everything that runs FS2/FSO.  So if German is set and your say try to run TBP which does not contain the language files you run into issues. 

[Satellight]

The hand-made french translation by petar91 works fine in all mods. He translated all the missions in FS2 directly in the files and partially the .tbl files for the interface and the technic room. Unfortunately I wasn't able to find a german fan-made translation in google, but an italian one exist. It's compatible with FSO...

BTW, for those interested :

French translation here : http://nebulastar.free.fr/download/freespace2/traduction/Freespace2_SCP_FR_v1.0.rar

Italian translation here : http://multiplayer.it/download/freespace-2-per-pc/freespace-2-traduzione-in-italiano-base-v10/

Multilingual translations will be a good (and HUGE ) project to start.